(1) Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary cell having a positive electrode comprising mainly of a positive electrode active material, a negative electrode, and a nonaqueous electrolyte, and to a method of manufacturing the nonaqueous electrolyte secondary cell.
(2) Description of the Prior Art
In recent years, nonaqueous electrolyte cells have attracted attention as cells capable of providing high capacity. The nonaqueous electrolyte cell uses, as the positive electrode material, a lithium-containing composite oxide such as a lithium cobalt oxide and uses, as the negative electrode material, a lithium-aluminum alloy, a carbon material or the like capable of intercalating and deintercalating lithium ions.
However, the lithium cobalt oxide is known to have low thermal stability in a charged state. In view of this, there is a known method such as modifying the synthesis conditions (for example, by increasing the calcination temperature or by increasing the calcination time) when preparing a lithium cobalt oxide, to increase the crystallite size of the (110) plane of the lithium cobalt oxide to 900 angstroms or more. However, a cell which used a lithium cobalt oxide synthesized by such a method suffered a great deterioration, when the cell was subjected to repeated charge/discharge cycles at high temperatures or when the cell was stored in a fully charged state, exhibiting poor high-temperature characteristics.
In order to overcome such a problem, there has been suggested a method (Patent Application No. 2001-100897) in which noticing the fact that there is a correlation between pH of filtrate collected after dispersing the positive electrode active material in water and high-temperature characteristics, LiF is added when synthesizing a lithium cobalt oxide, to lower the pH of the filtrate, thereby improving the high-temperature characteristics. In this method, in dramatically improving the high-temperature characteristics, the pH of the filtrate needs to be lowered to below 9.8. The lowering of the pH of the filtrate to such levels requires adding a large amount of halogen (fluorine) such that the halogen (fluorine) content in the total amount of positive electrode active material exceeds 5 mass %, which in turn causes a reduction in the cell capacity.